1. Field of the Invention
The present invention relates to a brushless motor control apparatus, a brushless motor and a control method of the brushless motor.
2. Description of Related Art
Some blower motors are formed as a multi-phase brushless motor. In a control apparatus of such a brushless motor, semiconductor elements (switching elements) are turned on and off to apply a voltage of a rectangular wave to each corresponding one of coils of multiple phases of the motor, so that a rotor of the motor is rotated. When the energization of one of the coils is turned off at the timing of changing the active phase of the motor to another phase, a return current flows through the corresponding coil to generate a torque ripple, thereby resulting in generation of annoying sound and vibration.
In view of the above disadvantage, a technique has been proposed to limit the generation of the sound and vibration by slowing down the change of the energization from one coil to another coil. For example, Japanese Patent No. 2692103B teaches a technique of shaping a rectangular wave signal into a bilaterally symmetric trapezoidal wave signal through use of a current, which is set based on a speed signal received from a rotational speed sensing circuit. Alternatively, Japanese Patent No. 2731647B teaches a technique of generating a drive current of a motor. According to this technique, a triangular wave, which matches with a rectangular wave, is generated by using an analogue technique. The thus generated triangular wave is combined with the rectangular wave to form a bilaterally symmetric trapezoidal wave, which is then used to generate the drive current. Further alternatively, Japanese patent No. 3611423B teaches a technique of generating a voltage of a bilaterally symmetric trapezoidal wave based on an output of an up/down counter that executes up-counting, holding or down-counting.
Furthermore, Japanese Patent No. 3854186B teaches a technique of generating a voltage of a bilaterally symmetric trapezoidal wave by a pulse width modulation (PWM) signal to more slowly or gradually change the energization of one coil to another coil at the time of turning off the energization of the coil.
In the case of the technique recited in Japanese Patent No. 2692103B, the motor, to which the above technique is applied, is a half-wave motor. When the technique of Japanese Patent No. 2692103B is applied to a full-wave motor, which is often used as a high-current motor, a circuit size is disadvantageously doubled. Furthermore, in the case of the technique recited Japanese Patent No. 2731647B, the circuit structure is disadvantageously complicated. In addition, in the case of the technique recited in Japanese patent No. 3611423B, the up/down counter needs to be provided to each of the phases of the motor.
Furthermore, the techniques recited in Japanese Patent No. 2692103B, Japanese Patent No. 2731647B and Japanese patent No. 3611423B, respectively, use a linear control scheme, which changes the voltage supplied to the motor, and is not suitable for the PWM control scheme that changes a time ratio of ON and OFF of the voltage supplied to the motor.
Furthermore, in the case of the technique recited in Japanese Patent No. 3854186B, a control apparatus, which uses a microcomputer, is used. Therefore, the circuit structure becomes disadvantageously complicated.